1. Field of the Invention
The present invention relates to a process for preparing reverse-osmosis membranes and to the membranes produced by said process. In particular, the field of the present invention relates to a process which is particularly suitable for forming planar membranes which are asymmetrical with respect to the plane which cuts the membrane halfway across its thickness. The field of the present invention further relates to polypiperazinamide-based membranes, which are defined as all polycondensation products of piperazine or of piperazine with alkyl substitutions in the nucleus, possibly in mixture with other diamines, with anhydrides or dichlorides of saturated or unsaturated, aromatic or heterocyclic aliphatic di-carboxylic acids, such as fumaric acid, mesaconic acid, adipic acid, phthalic acid, isophthalic acid, phthalic acids with substituted aromatic nucleus or heterocyclic acids derived from furan, thio-furan, pyridine, thiophene and the like, either alone or in mixture with one another. Polypiperazinamides belonging to the field of the present invention are described in the Italian patent application No. 22179 A/82 filed by Montedison S.p.A. which related to the production of ultrafiltration and reverse-osmosis membranes.
In particular, Italian patent application No. 22179 A/82 discloses a procedure for manufacturing asymmetrical planar supported membranes based on polypiperazinamides, wherein the polypiperazinamide is dissolved in a polar organic solvent, the obtained solution is applied on a planar or tubular support, the body thus formed being then gelled by being passed through a coagulation bath.
The above is summarily the main field of industrial use of the invention; said field however does not limit the scope of the present invention, since the process according to the invention, in particular as described and claimed hereinafter, can be advantageously used in any other equivalent field in which reverse-osmosis membranes are prepared by means of a solubilization step and a phase-reversal step.
2. Prior Art
In practice, however, it has been observed that the process illustrated in the Italian patent application No. 22179 A/82 is not free from disadvantages. In particular planar membranes thus obtained have scarce mechanical resistance, which prevents or at least severely complicates the formation of coiled-spiral modules, so that the membranes can be used only in the planar or tubular form in which they were originally formed. This constitutes a limitation in practical use, since the formation of coiled-spiral modules is in itself particularly useful and practical for the final use of the membranes.
The aim of the present invention is therefore to eliminate the above described disadvantages by providing a polypiperazinamide-based asymmetrical planar membrane having considerable mechanical resistance such as to allow the working of said planar membrane by folding to form coiled-spiral membrane modules, without the overall properties of the membrane suffering from any appreciable degradation.
An object of the invention is to form the membrane on a non-woven-fabric support in a planar configuration which can subsequently be folded to form coiled-spiral modules.